| In order to promote and accelerate the development of the optical system to the direction of the small volume,large capacity,low equipment costs,multifunction and high operating efficiency,the concept of integrated optics was proposed and its main goal is to achieve the miniaturization of optical systems.Several micro-nano photonic devices are fabricated and integrated on a single optical chip substrate to realize the processing and transmission of optical signals,thus forming an integrated optical circuit.Integrated photonic devices include passive and active photonic devices.A passive photonic device refers to the device that doesn't need an additional energy drive,such as beam splitters,couplers,filters,attenuators,polarizers and so on.Active photonic devices work with additional energy drives,such as lasers,amplifiers,modulator,etc.Optical waveguide,a micro-nano structure that confines and guides light waves through the total internal reflection at the interface of the dielectric material,is an important basic component of the above photonic devices.Therefore,the refractive index in the waveguide region is often higher than that of the surrounding medium,which enhances the optical power density in the waveguide.Benefitting from the compact size of optical waveguide structure,optical waveguide devices with different functions can be prepared(waveguide couplers,waveguide amplifiers,waveguide lasers,etc.),which can effectively realize optical circuit integration.So far,a variety of waveguide structures has been fabricated in dielectric crystals,such as planar waveguide structures,channel waveguide structures and cladding waveguide structures and so on.Up to now,how to fabricate optical waveguide structures in crystals with small volume,high efficiency,low loss and high stability is still an important basic research topic in integrated optics.In this paper,the planar waveguides are prepared in dielectric crystals by ion irradiation that has the advantages of high controllability,wide applicability and high repeatability.During ion irradiation,the incident energy ions collide with the nucleus or electrons of the crystal target and stop inside the crystal at a certain depth by exchanging energy with target atoms.Finally,the lattice arrangement of the impacted positions is changed so that the refractive index of this region is changed,resulting in a index difference between the impacted region and the region that do not impact.Ion irradiation techniques can not only prepare planar waveguides,but also combine with other micro-nano machining techniques such as photolithography,mask,and precision diamond blade dicing to prepare complex waveguide structures for different applications.Nanomaterial science dedicates to study properties and applications of materials at least one dimension less than 100 nm where the electrons must confine at least one direction in three-dimensional space.As a result,the interlayer interaction of the material in this direction disappears and the physical and chemical properties of the material will change,even if the elements that make up the material do not change.However,due to their the size effect,nnomaterials in different dimensions will exhibit different properties.If the motion of electrons in a material is restricted in two and three directions,there will be one-dimensional and zero-dimensional materials,respectively.If the motion of electrons is limited in one direction,a two-dimensional material can be obtained.The first two-dimensional material discovered experimentally is graphene that is the basic structure unit of the carbon materials family,which attracts great research interest in the field of two-dimensional materials.By cutting out different shapes of nanosheets from graphene,we can wrap zero-dimensional materials called fullerenes,roll one-dimensional materials called carbon nanotubes and stack three-dimensional materials called graphites.With the development of material preparation methods and characterization techniques,more and more two-dimensional materials with unique mechanical,electrical and optical properties that are different from multilayer and bulk counterparts have been found,including black phosphorus,boron nitride,transition metal dichalcogenides and a variety of two-dimensional heterostructures.Thus,they have a good application prospect in optoelectronic devices,solar cells,sensors and saturable absorbers.Due to the wide applications of two-dimensional materials,researchers are not only interested in their intrinsic properties,but also keep looking for appropriate modification methods to tailor them,discovering new properties and expanding their new applications.Ion beam technology is one of the most widely recognized methods for material modification.Ion irradiation is a controllable and stable defect engineering technology.By selecting different ion energies and doses,researchers can effectively introduce specific types and concentrations of defects into two-dimensional materials.As a result,properties of two-dimensional materials are modified.In addition,ion irradiation can effectively control the interlayer distance of two-dimensional heterostructures,leading to tightly attached layers.The heterostructure mechanically assembled together tends to have a large number of ripples,folds and wrinkles,resulting in a large interlayer distance and a low interlayer coupling in the heterostructure,which seriously affects its optical properties.When the energy ion beams interact with the two-dimensional heterostructure,the high-energy ions will transfer energy to the heterostructure and provide momentum for the heterostructure,which reduces the interlayer distance.As a result,the interlayer coupling of the heterostructure is enhanced.This dissertation mainly includes the fabrication of waveguide amplifiers in dielectric crystals and continuous-wave and Q-switched pulsed waveguide lasers in laser crystals by ion irradiation.In addition,the modification of monolayer two-dimensional materials and their heterostructures by ion irradiation is studied.According to different optical waveguide devices prepared and different structures of two-dimensional materials,the main research contents of this dissertation can be summarized as follows:A planar optical waveguide with a thickness of about 10 ?m was prepared by carbon ion(C5+)irradiation in an erbium and magnesium oxide-doped near stoichiometric lithium niobate(Er:MgO:SLN)crystal.Dark modes of the planar waveguide at 1539 nm were measured by a prism coupling experiment.The experimental results show that there were 7 guiding modes and the low-order modes were well confined.Then,two air grooves with a horizontal spacing of 25 ?m were cut on the Er:MgO:SLN planar optical waveguide by the precision diamond blade dicing.The waveguide layer between the grooves was the ridged optical waveguide.The refractive index distribution of the ridged waveguide was fitted by the reflectivity calculation method(RCM)combined with the planar waveguide's dark mode spectrum.The refractive index distribution of the ridged waveguide was found to be "optical barrier + enhanced well" and the ridged waveguide only support single-mode transmission in the near infrared region.Under a 980 nm laser exciting,the fluorescence properties of the Er:MgO:SLN ridged waveguide were well preserved and the greater than that in the Er:MgO:SLN crystal.Finally,using Er:MgO:SLN ridged waveguide as the gain medium,the signal light amplification in the communication C-band was realized under the 980 nm laser pumping.At the pumping power of 99.5 mw,the gain at the wavelength of 1536 nm,155 nm and 1565 nm were 2.13 dB/cm,1.49 dB/cm and 1.37 dB/cm,respectively.A cladding channel waveguide with width of 40 ?m was fabricated in the ytterbium-doped yttrium scandium gallium garnet(Yb:YSGG)crystal by twice carbon ion(C5+)irradiations combined with the precision diamond blade dicing.Experimental results show that due to the superposition effects of ion irradiation,a cladding channel waveguide structure was formed on the Yb:YSGG crystal surface(core layer-inner cladding layer-out cladding later).The refractive index distribution of the cladding waveguide was found to be a graded index:the refractive index of the core is the largest,while the refractive index of the inner and outer layers was smaller than that of the core layer.Using the Yb:YSGG cladding channel waveguide as the gain medium,the 1023.6 nm continuous-wave waveguide laser oscillation was realized under the 940 nm laser pumping.The maximum output power and slope efficiency were 52.3 mw and 46%,respectively.The Q-switched pulsed waveguide laser with a wavelength of 1024.8 nm was obtained using tungsten disulphide(WS2)nanosheets as a saturable absorber coated on the surface of the Yb:YSGG cladding waveguide.The minimum pulse duration was 125 ns under the 227 mW laser pumping.Graphene and vanadium dioxide(VO2)were combined with Nd:YAG crystal waveguides to form hybrid waveguide structures and saturable absorption properties of graphene and VO2 were tailored by electricity and heat,respectively.Under 810 nm laser pumping,1 ?m the continuous-wave and Q-switched pulsed waveguide lasers output were realized.Specifically,using the graphene-Nd:YAG hybrid waveguide as the gain medium,a Q-switched pulsed laser was obtained with a wavelength of 1064 nm at 808 nm laser pumping without voltage signal.The maximum average output power was 15 mW under 500 mW laser pumping,corresponding pulse duration(repetition frequency)of 32 ns(2 MHz).When the voltage increased from 0 V to 8 V,it can be observed that the output pulse profile gradually widened and the pulse duration increased from 32 ns to 260 ns.When the input voltage was higher than 10 V,the continuous-wave laser was obtained.In addition,as the voltage increased from 0 V to 14 V,the modulation depth of graphene slowly decreased from 1.2%to 0%,indicating that the saturation absorption of graphene can be controlled by applying positive voltage.The above results show that the hybrid graphene-Nd:YAG waveguide laser output is electrically tuned between the continuous-wave waveguide laser and the nanosecond pulsed waveguide laser.When a current signal was applied in Nd:YAG waveguide laser system,the output laser power firstly decreased slowly from 10 mW to 7 mW and then rapidly to zero with the increase of the current value.At the same time,the pulse duration increased from 40 ns to 138 ns,indicating that the current can control the pulsed waveguide laser in the on and off state.Under the 810 nm laser pumping,the 1064 nm Q-switched VO2-Nd:YAG hybrid waveguide laser was achieved,with a maximum average power output of 47 mW and a minimum pulse duration of 2.3 ns.By controlling the temperature of VO2,the saturable absorption of the VI2-Nd:YAG hybrid waveguide was observed during transition from the insulator to the metallic phase of VO2.When VO2 film was heated to 325 K,700 ps pulsed laser output was obtained in the VO2-Nd:YAG hybrid waveguide.The VO2 film was further heated to 330 K,and the output laser is transferred from the pulsed laser to the continuous-wave laser.With the increase of the temperature,the maximum output power of the continuous-wave laser was 37 mW.When the VO2 film was cooled to 312 K,pulse trains were observed.In the meanwhile,the pulse duration and average output power rapidly increased to room temperature values.Compared to the heating process,it can be found that the performance of Q-switched pulsed laser showed thermal hysteresis during the cooling process.Ion irradiation was used to irradiate tungsten disulfide(WS2)monolayers and graphene and tungsten selenide(Graphene/WSe2)heterostructures,respectively.The optical properties and applications of those irradiated two-dimensional materials were studied.Specifically,WS2 monolayers deposited on sapphires were irradiated by argon ions(Ar+)at the energy of 60 keV at different ion doses.The irradiated samples were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy(XPS).The results show that there were a large number of sulfur and tungsten vacancies in the irradiated WS2 and the density of those vacancies was related to the irradiation dose:the larger the radiation dose,the greater the concentration of sulfur vacancies,which means that the ratio of sulfur atoms to tungsten atoms in WS;2 decreased with the increase of the radiation dose.First-principles calculations indicate that there were intermediate electronic states in the band gap of defective WS2 and the density of electronic states increased with the increase of sulfur vacancy concentration.When the ratio of sulfur atoms to tungsten atoms was 1.899,WS2 can absorb approximately 1 eV near-infrared light.In addition,the linear optical absorption and the saturated absorption of WS2 in the near infrared range also increased with the increase of sulfur vacancy density.The irradiated WS2 used as a saturable absorber was integrated into the Nd:YAG waveguide laser systems to realize and optimize the near-infrared passively Q-switched pulsed lasers.Graphene/WSe2 heterostructures deposited on the sapphire and the copper grid were irradiated by carbon ions(C3+)at the energy of 6 MeV and applied as a surface-enhanced Raman scattering(SERS)substrates.Using the copper phthalocyanine(CuPc)molecule as a probe,the intensity of the Raman scattering and SERS enhancement mechanism of Graphene/WSe2 were investigated.The experimental results show that the intensity of the Raman scattering Graphene/WSe2 was much stronger compared with the isolated layers.The Graphene/WSe2 SERS substrate achieved an enhancement factor of 28.6(1528.3 cm-1 Raman mode)and the SERS signal was uniform.In addition,the SERS activity of Graphene/WSe2 was not affected by the Al2O3,but depended heavily on the stacked order of heterstructures.The first-principles calculations and pump-probe measurments display that because of the tight contact between Graphene and WSe2 in the irradiated Graphene/WSe2,there was a strong interlayer coupling in the heterostructure,which promoted the increase of interlayer charge transfer.When the irradiated Graphene/WSe2 heterostucture was used as a SERS substrate,the ultrafast interlayer charge transfer provided the enriched electronic density of states around the probe molecules,increasing the intensity of Raman scattered light of probe molecules. |
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